Until recently the specifications provided for X-ray examination systems by suppliers to the tender evaluation process has been minimal. This information typically includes only:                single wire resolution in air (perhaps, 32 American wire gauge (AWG))        penetration of steel (perhaps 90 mm for a 450 kiloelectron volts (keV) system)Reading onwards in the specifications one might find that the system is capable of materials discrimination and even automatic explosives detection, and that software exists to enable this to be done.        
No standard test piece for the evaluation of the performance of medium and large tunnel (pallet and container) X-ray examination systems presently exists.
Since 2002, however, the Australian Customs Service (ACS) has, for cabinet machines (tunnel sizes of 1000 mm×1000 mm, and operating X-ray energies of around 160 keV) been using data collected form the examination of an American Society for Testing and Materials (ASTM) F 792-01E2 standard suitcase to determine objectively the performance of the X-ray system. FIG. 1 shows the construction of the standard suitcase, and FIG. 2 is an X-ray image taken of the system using a dual energy cabinet X-ray system operating at 140 keV.
The function of each of the various tests is set out in Table 1.
TABLE 1The function of each of the tests in the standard suitcase are noted.TESTCOMMENT1Check for visibility of wires in air (24 to 38 AWG)2Check for visibility of TEST 1 wires behind a step wedge(9, 5, 15.9, 22.2 mm)3Check resolution of wire bundles (bundles of four wireswith the wires in each bundle being 2, 1.6. 1.4. 1.0 mm)4Penetration test (steel stepwedge, lead letters denoting the wedge thickness in mm)5Plastic samples of the same thickness, which must be discriminated one from the other6Contrast sensitivity tests for plastic and metal blocks. Holes of different diameters and depths in the blocks must be identified.7Materials discrimination: the system must discriminate between plastic and metal objects with the same X-ray attenuation.8Materials discrimination: discrimination of adjacent organic objects.9Materials discrimination: discrimination of organic objects behind steel step wedges of thickness, 0, 0.16, 0.32, 9.48 mm
A standard score sheet exists for evaluating the performance of the system (FIG. 3), and instructions are given to enable the score sheet to be filled out.
The performance of this system was evaluated to be 58%. It performed badly on Test 2, Test 3, half of Test 6, and did not meet manufacturer's specification for Test 1 and Test 4.
ASTM F 792-01E2 is a small case designed for small systems (e.g. hand luggage, or for machines with tunnel sizes up to 1000 mm wide) and is not useful for pallet and larger systems. It is also difficult to use as it has no angle adjustments to account for scanner contraction geometries. Equivalent test objects and score sheets do not exist for X-ray systems with tunnel to sizes greater than 1200 mm×1200 mm (pallet examination systems and exist). Several suppliers have given to the Australian Customs Service collections of rods, wires, and plates with minimal instruction as to how these are to be used to evaluate system performance. FIG. 4 shows one such arrangement, used for testing a dual view, dual energy 2.5 megaelectron volts (MeV) system at an ACS Container Examination Facility. An attempt has been made to compensate for the fact is that the axes of the horizontal and vertical X-ray fans are inclined at an angle to the horizontal.
To undertake the CS (Contrast Sensitivity) and IQI (Image Quality Index) tests plates were removed one at a time from the vertical plate stack and placed on the horizontal stack. The object had to be passed through the X-ray system each time the plate thicknesses were changed. The time taken to complete the examination procedure was in excess of six hours. Results for a single pass through the system are shown in FIG. 5.
It can be readily seen that the angle guessed for the X-ray beam inclination was too small as indicates by the black images of the bars which carry the plates (FIG. 5(a)). Interference from the plastic pallet on the images of the test wires can be seen in FIG. 5(b).
As well, conducting a penetration test using the vertical beam is compromised because of the fact that the “vertical” and “horizontal” beams are at least 20° inclined to vertical or horizontal lines which means that the beam path is increased, and the supporting pallet produces artefacts in the image. (FIG. 5(c)). The lead block (100 mm×100 mm×100 mm) is barely visible at the manufacturer's specification of 180 mm. An advantage of being able to tilt the test object is to place it on a true perpendicular intercept by the beam and minimise distortions and is inaccuracies induced by the interaction angle not being 90° to the beam (i.e. a kind of a parallax error). This is illustrated in FIG. 5d. 
There is a need for a test piece for use in for X-ray systems with large tunnel sizes, e.g. greater than about 1000 mm×1000 mm.